The present invention relates to a pulse generating circuit having simultaneous amplitude modulation and pulse width modulation, and in particular to such a circuit utilized as a brass keyer in electronic organs and the like.
As is well known, the tonal quality of a note produced by a musical instrument is dependent upon the harmonic content of the waveform of the note. In electronic organs, it is customary to generate square waves, which are then altered or filtered to produce waveforms having the desired harmonic content. The duty cycle of the square waves has a very pronounced effect on harmonic content, with pulse trains having a short duty cycle, i.e. a narrow pulse width, sounding more brilliant, and pulse trains having a longer duty cycle, i.e. a wider pulse width, sounding more mellow. An example of the former, is the sound produced by a trumpet, and an example of the latter is the sound produced by a saxophone.
A characteristic of the sound produced by brass instruments and some woodwind instruments is that during the attack portion of the tone, the amplitude increases and the tonal quality becomes more brilliant, i.e. the duty cycle decreases. This sound may be simulated in a electronic organ by causing the tone to gradually build in amplitude when the key is depressed with a gradual decrease in duty cycle. The duty cycle may either decrease simultaneously with the increase in amplitude, or lag the amplitude build up somewhat so as to simulate the effect which is produced when a mute is used with the trumpet or trombone. During decay, the pulse amplitude will decrease with the duty cycle becoming progressively longer. A small amount of white noise may be injected into the tone in order to simulate the breathy quality of brass tones.
Although the ability to simulate brass sounds is a desirable feature on electronic organs, the facts that a separate keyer is normally required for each note having brass voicing capability and the fact that a relatively large number of discreet components are necessary to produce the pulse width modulation necessary for brass voicing, have generally limited the capability for brass voicing to the more expensive organs. Furthermore, it is often difficult to adjust the various parameters of the brass keyers so as to properly balance the amplitude modulation and pulse width modulation so that separate keyers would often be required for the brass, trombone, saxophone, etc. voices.